Integrated debris management system

ABSTRACT

A fluid filtration system of well workover fluids, the system comprising: a first settling tank which stores fluid; a second settling tank which stores fluid; a low pressure pump in fluid communication with the first and second settling tanks; a low pressure filter in fluid communication with the first and second settling tanks; a high pressure pump in fluid communication with the second settling tank; and a high pressure filter in fluid communication with the second settling tank, wherein the high pressure filter comprises: an inlet; an outlet; a plurality of input feed lines in fluid communication with the inlet; a plurality of output feed lines in fluid communication with the outlet; a plurality of filter canisters, wherein at least one filter canister is in fluid communication with each input feed line and each output feed line.

BACKGROUND OF THE INVENTION

[0001] The present invention relates generally to a system and methodfor removing solid debris from completion fluids being pumped through asurface drilling rig circulation system prior to flowing downhole.

[0002] In the prior art, when debris is pumped or introduced downhole,such debris or trash becomes lodged in downhole equipment or productiontools and impairs their performance or causes failure. Free floatingdebris such as drilling mud solids, paint chips, pipe dope, metalshavings, rust shale, fibers and other associated debris are typicallyfound in the completion fluids. Accumulation of this collective array ofdebris will lead to equipment failure as previously noted.

[0003] As shown in FIG. 1, prior art workover systems typically comprisefirst and second settling tanks 2 and 3, a low pressure pump 4, a lowpressure filter 5, and a high pressure pump 6. The low pressure pump 4and low pressure filter 5 transport and purify the workover fluid fromsettling tank 2 to settling tank 3. A high pressure pump 6 drawsworkover fluid from settling tank 3 for insertion into the wellChristmas tree 8 located at the top of the wellbore casing 7. A highpressure conduit 10 extends from the high pressure pump 6 to theChristmas tree 8. The systems further comprise a return conduit 9 whichextends from the Christmas tree 8 to the first settling tank 2.Relatively dirty completion fluid is drawn from the Christmas tree andpumped into the first settling tank 2, where the bulk of the debrissettles out of the fluid. Completion fluid is then pumped out of the topof the first settling tank 2 so as to withdraw as little of the debrisas possible with the completion fluid. As the completion fluid iswithdrawn from the first settling tank 2, it is filtered by the lowpressure filter 5 to remove any debris suspended in the completionfluid. This filtered completion fluid is pumped into the second settlingtank 3 to allow any remaining debris to settle out of the fluid.

[0004] Low pressure filtration systems for implementation between thefirst and second settling tanks 2 and 3 are known. For example, U.S.Pat. No. 4,352,739, incorporated herein by reference, discloses a liquidcleaning system for liquid that is substantially continuously circulatedand which becomes contaminated with solid particulate and debris duringcirculation. The liquid cleaning system, which is particularlyadvantageous to cleaning liquid that is circulated within a petroleumwellbore during completion and workover activities, incorporates avibratory screen separator for accomplishing preliminary separation oflarge particulate and debris from the contaminated liquid and which alsoincorporates a filtration bank having a plurality of filters that areeach interconnected with inlet discharge and drain headers. The filterbank incorporates a valving system having the capability ofaccomplishing backflushing by circulating clean fluid in reverse mannerthrough selected ones of the filters to remove collected solidparticulate therefrom and transport the removed particulate to the drainheader.

[0005] Similarly, U.S. Pat. No. 4,560,483, incorporated herein byreference, discloses a filtering system using at least two filtervessels with a plurality of filter elements having absolute filtrationcapability. The filter elements use a nominal filter of diatomaceousearth or a perlite filter and a precoat layer, an outer wire layer, anabsolute filter layer, a fine wire layer, a course wire layer, and atube core for receiving filter flow therein. A pump is provided forinjecting a predetermined amount of filter aid to a precoat tankcontaining clean fluid. The slurry of filter aid and clean fluid is thenpumped into one of the filter vessels for precoating the filterelements. The unfiltered fluid is then allowed to enter the vessel andto be filtered through the filtering elements while virgin filter aid iscontinuously injected into the unfiltered fluids prior to the fluidsentering the filter vessel. This apparatus is used for filteringcompletion and workover fluids utilized during oil and gas well drillingand completion operations.

[0006] Another example is found in U.S. Pat. No. 4,456,061, incorporatedherein by reference. This patent discloses well cleaning circulationfluids and production or workover processes. In particular, acylindrical housing is adapted to receive and discharge fluid in asubterranean well. In a preferred form, a plate is positioned in thecylinder separating it into first and second chambers. Filter adapterreceiving means are disposed across the plate, with a filter adaptersecurable therein, the adapter having a passageway extending to thechambers. The adapter has a cylindrical housing having an enlargedcartridge positioning portion. A grooveway is defined on the adapterextending exteriorly therearound for securing one end of a secondarycartridge filter. A secondary cartridge filter has an inner filtercompletely surrounded by a replaceable and reuseable primary sleevefilter and is positionable on the adapter, the filter providing a sealhaving an interiorly facing circumferentially extending lip for contactwith the exterior of the cylindrical housing when the filter is locatedon the adapter, for sealingly securing the filter to the adapter.

[0007] While many filter systems are known throughout the industry forfiltering the circulation fluid in the settling tanks, these systems arenot completely effective in removing debris from the completion fluid asthe fluid is pumped into the wellbore. In particular, because thesettling tanks are sometimes located several hundred feet away from thewell being worked over, the high pressure conduit 10 can retain a fairamount of debris which is carried by the completion fluid into the well.Therefore, there is a need for a filtration system and method which moreeffectively removes debris and particulates from the completion fluidprior to the fluid being introduced into the well.

SUMMARY OF THE INVENTION

[0008] To satisfy this need in the well completion fluid circulationfield, a high pressure filtration system is placed in the high pressureconduit immediately upstream from the Christmas tree. This high pressurefiltration system cleans fine particulates and debris from thecirculation fluid as high volumes of circulation fluid rapidly passthrough the filtration system under high pressure.

[0009] According to one aspect of the invention, there is provided afluid filtration system of well workover fluids, the system comprising:a first settling tank which stores fluid; a second settling tank whichstores fluid; a low pressure pump in fluid communication with the firstand second settling tanks; a low pressure filter in fluid communicationwith the first and second settling tanks; a high pressure pump in fluidcommunication with the second settling tank; and a high pressure filterin fluid communication with the second settling tank, wherein the highpressure filter comprises: an inlet; an outlet; a plurality of inputfeed lines in fluid communication with the inlet; a plurality of outputfeed lines in fluid communication with the outlet; a plurality of filtercanisters, wherein at least one filter canister is in fluidcommunication with each input feed line and each output feed line.

[0010] According to a further aspect of the invention, there is providedan in-line, high-pressure, fluid filter of well workover fluids, thefilter comprising: an inlet; an outlet; and a high pressure filtercanister in fluid communication with the inlet and the outlet, thefilter canister comprising a perforated base pipe and a media screen.

[0011] According to still anther aspect of the invention, there isprovided a high pressure fluid filter of well workover fluids, thefilter comprising: an inlet; an outlet; and a high pressure filtercanister in fluid communication with the inlet and the outlet, thefilter canister comprising a perforated base pipe and a media screen; aplurality of input feed lines in fluid communication with the inlet; aplurality of output feed lines in fluid communication with the outlet;and a plurality of high pressure filter canisters, wherein at least onefilter canister is in fluid communication with each input feed line andeach output feed line.

[0012] According to another aspect of the invention, there is provided aprocess for filtering fluid for circulation in a well, the processcomprising: standing the fluid a first time, whereby debris in the fluidis allowed to settle; filtering under a first pressure the fluid of thestanding the fluid a first time, whereby further debris is removed fromthe fluid; standing the fluid a second time, whereby further debris isallowed to settle; and filtering the fluid of the standing the fluid asecond time under second pressure, wherein the second pressure is higherthan the first pressure, whereby further debris is removed from thefluid.

[0013] The high pressure filter of the present invention withstands highworking pressures and also withstands high velocity erosion associatedwith high debris loading. Debris such as barite from drilling mud, fracsand or bauxite pumped at high rates could erode or wear a hole in themetal filter canisters leading to plugged or serious reduction ofhydrocarbon formations with the producing reservoir.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The present invention is better understood by reading thefollowing description of non-limitative embodiments with reference tothe attached drawings wherein like parts in each of the several figuresare identified by same reference characters, and which are brieflydescribed as follows:

[0015]FIG. 1 is a schematic depiction of a completion and/or workoversystem as is known in the prior art.

[0016]FIG. 2 is a schematic depiction of a filtration system for acompletion and/or workover system according to the present invention.

[0017]FIG. 3 is a schematic representation of a saddle manifold forconnecting the filtration system to the high pressure conduit betweenthe high pressure pump and the Christmas tree.

[0018]FIG. 4A plan view of the high pressure filtering device of thepresent invention.

[0019]FIG. 4B is an inlet elevation of the high pressure filteringdevice of the present invention shown in FIG. 4A.

[0020]FIG. 4C is a side elevation of the high pressure filtering deviceshown in FIGS. 4A and 4B.

[0021]FIG. 4D is an outlet elevation of the high pressure filteringdevice shown in FIGS. 4A-4C.

[0022]FIG. 5 is a cross-sectional side view of a filter canister of thepresent invention.

[0023] It is to be noted, however, that the appended drawings illustrateonly typical embodiments of this invention and are therefore not to beconsidered limiting of its scope, as the invention may admit to otherequally effective embodiments.

DETAILED DESCRIPTION OF THE INVENTION

[0024] Referring to FIG. 2, a schematic diagram of an embodiment of thepresent invention is shown. A first settling tank 2 is shown containingcirculation and/or completion fluid. A low pressure pump 4 drawscirculating fluid from the first settling tank 2 and pumps it through alow pressure filter 5. This filtered circulating fluid is deposited in asecond settling tank 3. A high pressure pump 6 draws circulating fluidfrom the second settling tank 3 and pumps it through a high pressureconduit toward the Christmas tree 8. Immediately upstream from theChristmas tree 8, there is a high pressure filter 20 connected in linewith the high pressure conduit 10. A return conduit 9 extends from theChristmas tree 8 to the first settling tank 2 for returning circulationfluid from the wellbore casing 7 to the first settling tank 2.

[0025] As shown in FIG. 3, according to one embodiment of the invention,a saddle manifold system 15 is used to connect the high pressure filter20 to the high pressure conduit 10 (not shown). In one embodiment of theinvention, the saddle manifold system 15 is a high pressure junctiondevice constructed of 5″, 3″ and 2″ pipe that is fitted and welded tothe high pressure conduit 10. The saddle manifold system 15 providesmultiple flow communications of completion fluids to and from the highpressure filter 20 through the saddle manifold system 15 and downholethrough the Christmas tree 8.

[0026] Fluid manipulation or stream flow direction is achieved with highpressure valves as shown in FIG. 3. Debris filled completion fluids flowfrom the high pressure pump 6 (see FIG. 2) through the high pressureconduit 10 to the saddle manifold system 15.

[0027] A bypass stand pipe 51 of the saddle manifold system 15 isconnected at one end to the high pressure conduit 10 and at the otherend it is capped with a cap 61. The saddle manifold system 15 also has afilter stand pipe 52, wherein a cross pipe 53 connects the two standpipes 51 and 52. The bypass stand pipe 51 has valves 54 and 55, whereinthe valves are positioned opposite the junction with the cross pipe 53,respectively. Similarly, the filter stand pipe 53 has 56 and 57positioned opposite the junction with the cross pipe 53, respectively.The filter stand pipe 52 is connected to the high pressure conduit 10 atone end and is capped with a cap 61 at the other end. Fluid flows fromthe filter stand pipe 52 to the Christmas tree (see FIG. 2) through anexit junction 62. The cross pipe 53 has a filter junction 58 forconnecting the cross pipe 53 to the high pressure filter 20. The crosspipe 53 also has two valves 59 and 60 which are positioned opposite thefilter junction 58. The cross pipe 53 also has a test line 63 with atest valve 65 and a fill up line 64 with a fill up valve 66. The standpipes 51 and 52 also each have a union 67, a plug 68, and a plugged line69, all of these being useful for flushing and monitoring procedures.

[0028] In one embodiment of the invention, the stand pipes 51 and 52 are5″ pipes and the cross pipe 53 is a 3″ pipe. The test line 63, fill upline 64, unions 67 and plugs 68 are all 2″.

[0029] The saddle manifold system 15 operates by opening the valve 53 onthe bypass stand pipe 51 and closing the valve 55 of the same pipe. Thefluid flows into the cross pipe 53 through the open valve 59 and intothe filter junction 58. The valve 60 on the cross pipe 53 is closed toforce the fluid from the filter junction 58 into the high pressurefilter 20. In one embodiment, fluid flows through the high pressurefilter 20 by way of 3″ chicksans lines rated to 15,000 psi. and thenfiltered clean fluid is feed to the filter stand pipe 52 by 3″ chicksansto the exit junction 62. Note that the valves 56 and 57 located aboveand below the T manifold on filter stand pipe 52 are closed to preventfluids from flowing down or into the filter stand pipe 52. The test line63 and fillup line 64 are test lines and feed lines that allow the highpressure filter 20 to be back flushed.

[0030] Referring to FIGS. 4A-4D, an embodiment of the high pressurefilter 20 is described. The high pressure filter 20 comprises an inlet21 and an outlet 22. The high pressure filter 20 also has, according tothis embodiment, eight filter canisters 23. As shown in the drawings,the filter canisters 23 are oriented vertically. The inlet 21 isconnected to the tops of the filter canisters 23 by two input feed lines24. The input feed lines are each equipped with an input valve 25. Theoutlet 22 is connected to the bottoms of the filter canisters 23 by twooutput feed lines 26. The output feed lines 26 are each equipped with anoutput valve 27. The high pressure filter 20 also comprises a by-passline 28 which extends from the inlet 21 to the outlet 22. the by-passline 28 also has a by-pass valve 29.

[0031] The high pressure filter 20 operates by flowing completion fluidsinto the inlet 21. In a filter configuration, the by-pass valve 29 isclosed and the input and output valves 25 and 27 are all open. In thisconfiguration, completion fluid flows from the inlet 21, through theinput feed lines 24, through the filter canisters 23, through the outputfeed lines and out the outlet 22. In a by-pass configuration, theby-pass valve 29 is open and the input and output valves 25 and 27 areall closed. In this configuration, completion fluid is prevented fromflowing through the filter canisters 23 and is instead routed throughthe by-pass line 28.

[0032] A further valve configuration allows an operator to keep the highpressure filter 20 online (filtering the completion fluid) andsimultaneously change out a filter in a filter canister 23 in case afilter becomes plugged. For example, if a filter canister 23 becomesplugged, the side of the high pressure filter 20 having the pluggedfilter canister 23 may be closed while the other side remains open. Inparticular, on the side of the high pressure filter 20 with the pluggedfilter canister 23, the input valve 25 and output valve 27 are closed toisolate all of the filter canisters 23 on that side. The remainingfilter canisters 23 on the other side of the high pressure filter 20remain open and flowing to filter the completion fluid. This enables anoperator to replace a plugged filter in a filter canister 23 withouttaking the entire high pressure filter 20 offline.

[0033] In alternative embodiments of the invention, the high pressurefilter 20 has more than two sides. For example, an alternativeembodiment has three or more input feed lines 24 and three or moreoutput feed lines 26. Further, while four filter canisters 23 are shownin the illustrative embodiment above, in alternative embodiments of theinvention, any number of filter canisters 23 are placed in the systembetween an input feed line 24 and an output feed line 26.

[0034] Referring to FIG. 5, a side cross-sectional view of filtercanister 23 of the present invention is shown. The filter canister 23contains a complete filter system having various components. Theembodiment shown in FIG. 5 has a base pipe 30 with a hex nut 31 weldedon one end and a threaded section 32 formed on the other. A base pipeseal 33 is also located on the base pipe 30 on the same end as thethreaded section 32. The base pipe seal 33 comprises a groove in thebase pipe 30 with an O-ring placed in the groove. The base pipe 30 alsocomprises perforations or holes 34 at various points along the length ofthe base pipe 30. A media screen 35 is applied to the exterior of thebase pipe 30. The threaded end of the base pipe 30 is threaded into anoutput T-pipe 36. The output T-pipe 36 has two sections, the T-pipe 37and the threaded flange 38, wherein the threaded flange 38 has bothexterior male threads and interior female threads. As shown in FIG. 5,the threaded section 32 of the base pipe 30 is threaded into the femalethreads of the threaded flange 38 of the output T-pipe 36. The base pipeseal 33 forms a seal between the base pipe 30 and the output T-pipe 36.

[0035] The filter canister 23 also has an exterior housing 39 with abeveled flange 40 at one end and a threaded shoulder 41 on the other.The exterior housing 39 has an output lock ring 42 around its outsidewhich has a shoulder for engaging the beveled flange 40. The insidediameter of the exterior housing 39 is larger than the outside diameterof the media screen 35 so that the exterior housing 39 can be slippedover the base pipe 30. With the base pipe 30 threaded to the outputT-pipe 36, the exterior housing 39 is slid over the base pipe 30 untilthe beveled flange 40 engages the threaded flange 38 f the output T-pipe36. The output lock ring 42 threads onto the male threads of thethreaded flange 38 of the output T-pipe 36 to secure the exteriorhousing 39 to the output T-pipe 36. An output seal 43 is sandwichedbetween the beveled flange 40 and the threaded flange 38 to ensure acompletely sealed connection.

[0036] The top of the filter canister 23 is closed with a flanged cap 44and an input lock ring 45. An input seal 46 is placed between theflanged cap 44 and the threaded shoulder 41 of the exterior housing 39and the input lock ring 45 is threaded onto the threaded shoulder 41 tocompletely seal the top of the filter canister 23. The flanged cap 44also has a handle 47 on top for ease of handling.

[0037] The input feed line 24 is connected to the filter canister 23 atan input hole 48 cut in the side of the exterior housing 39. The inputfeed line 24 is welded to the exterior housing 39 to complete theconnection.

[0038] In one embodiment of the invention, the exterior housing has a5¼″ outside diameter with a 4¼″ inside diameter. The media screen is aWeatherford 6 or 8 gauge wire wrap completion screen applied to a 2⅜″base pipe. This wire wrap completion screen is fusion welded tolongitudinal wire ribs of stainless steel metallurgy. The ribs provide astandoff for the wire wrap from the perforated pipe base to which thescreen is afixed.

[0039] In some embodiments of the invention, the high pressure filter 20utilizes three different types of filtration removal screens to insureadequate removal of a variety of particle sizes and volume. Designcriteria for pressure requirements of a working unit are about 7,500 psiwith a working pressure of about 500 psi. The high pressure filter 20should handle a volume output of about 25 barrels per minute to insuresufficient annular velocity for hole cleaning. The high pressure filter20 also has the ability to backflush and clean the filter surfacebecause of the large amount of debris that can be present on a workingdrilling rig and displaced debris from a wellbore.

[0040] In some embodiments of the invention, the high pressure filter 20has a series of steel cylinders designed to screw or set in placespecially engineered canisters to extract a wide range of oil fielddebris or trash from the rig circulation system. In some embodiments ofthe invention, three types of screen design are used to meet thefollowing needs: (1) a 12 gauge stainless wire wrap about a machinedblank pipe to remove very course or large sized debris; (2) a 0.0825micron metal screen attached to a blank pipe to facilitate removal offiner particles that would pass the larger 12 gauge canisters; and (3) a0.0425 micron metal screen to provide unique filtration of completionfluids prior to gravel packing or fracture packing a well. In analternative embodiment three screens are used: a stainless steel wirewrap screen about a machined perforated blank pipe to remove very coarseor large size debris; a wire mesh attached to a perforated blank pipe tofacilitate removal of finer particles that would pas the wire wrap; anda finer wire mesh screen filter device to provide unique filtration ofcompletion fluids prior to gravel packing or frac packing a well.Particle sizes that could pass through the larger filter devicesdescribed above, could impair the performance of sand controlprocedures, if placed with gravel sand. In some embodiments of theinvention where difference screen sized are implemented, multiple highpressure filters 20, such as the one illustrated in FIGS. 4A-4D areconnected in series starting with the high pressure filter 20 having themore coarse screen size and working to the filter having the finerscreen size.

[0041] It has been expressed that particle sizes that could pass throughthe larger filter devices described above, could impart the performanceof sand controlled procedures, if placed with gravel sand.

[0042] In one embodiment of the invention, the high pressure filter 20is built upon a 3′ 6″×5′ 6″ platform with eight canister housings thatare 4′ 6″ high. The inlet 21 and outlet 22 connections are 3″ NPT to 3″piping to accommodate high volume, high-pressure fluid circulation. Thecanisters 23 are fitted with hammer unions to allow quick opening andremoval or cleaning of the vessels and filtering devices.

[0043] Two or more filter racks, similar to the filter racks shown inFIGS. 4A-4D, may be incorporated into the system. This allows for onefilter racks to be operational while another filter rack is taken offlife for regeneration. A filter rack is regenerated by back flushingfluid through the filter or by removing the filter from the canister andrinsing the filter prior to reinsertion.

[0044] According to one embodiment of the invention, the high pressurefilter 20 is an inline filter device which fits within a 7″ casing. Itsets upright to the derrick support beam. Fluid flows from chokemanifold to standpipe and is not located in the work string. It has apressure rating of about 5000 psi. Depending on the particularapplication, the high pressure filter 20 has a screen with the abilityto back flush or clean easily such as a well water type screen. Debriscapacity and flow rates through the device are criteria used to selectthe proper screen for the given application. High pressure valvescontrol flow and/or isolate the filter from the standpipe. Sample valvesare placed proximate to the system to monitor debris in the stream anddown stream of the filter. A protocol for evaluating efficiency is usedto enhance performance for the given application.

[0045] While the particular embodiments for Integrated Debris ManagementSystems as herein shown and disclosed in detail are fully capable ofobtaining the objects and advantages hereinbefore stated, it is to beunderstood that they are merely illustrative of the preferredembodiments of the invention and that no limitations are intended by thedetails of construction or design herein shown other than as describedin the appended claims.

What is claimed is:
 1. A fluid filtration system of well workoverfluids, said system comprising: a first settling tank which storesfluid; a second settling tank which stores fluid; a low pressure pump influid communication with said first and second settling tanks; a lowpressure filter in fluid communication with said first and secondsettling tanks; a high pressure pump in fluid communication with saidsecond settling tank; and a high pressure filter in fluid communicationwith said second settling tank.
 2. A fluid filtration system as claimedin claim 1, wherein said high pressure filter comprises: an inlet; anoutlet; and a filter canister in fluid communication with said inlet andsaid outlet, the filter canister comprising a perforated base pipe and amedia screen.
 3. A fluid filtration system as claimed in claim 2,further comprising a by-pass line in fluid communication with said inletand said outlet.
 4. A fluid filtration system as claimed in claim 1,wherein said high pressure filter comprises: an inlet; an outlet; aplurality of input feed lines in fluid communication with said inlet; aplurality of output feed lines in fluid communication with said outlet;a plurality of filter canisters, wherein at least one filter canister isin fluid communication with each input feed line and each output feedline.
 5. A fluid filtration system as claimed in claim 4, wherein eachfilter canister of said plurality of filter canisters comprises aperforated base pipe and a media screen.
 6. A fluid filtration system asclaimed in claim 4, wherein said plurality of filter canisters comprisea plurality of media screens, wherein at least two of said plurality ofmedia screens are different.
 7. A fluid filtration system as claimed inclaim 6, further comprising a by-pass line in fluid communication withsaid inlet and said outlet.
 8. A fluid filtration system as claimed inclaim 1, wherein said high pressure filter is an inline filter in directfluid communication with said high pressure pump.
 9. A high pressurefluid filter of well workover fluids, said filter comprising: an inlet;an outlet; and a high pressure filter canister in fluid communicationwith said inlet and said outlet, the filter canister comprising aperforated base pipe and a media screen.
 10. A high pressure fluidfilter as claimed in claim 9, further comprising a by-pass line in fluidcommunication with said inlet and said outlet.
 11. A high pressure fluidfilter as claimed in claim 9, wherein said filter is an in-line filter.12. A high pressure fluid filter as claimed in claim 9, wherein saidhigh pressure filter further comprises: a plurality of input feed linesin fluid communication with said inlet; a plurality of output feed linesin fluid communication with said outlet; a plurality of high pressurefilter canisters, wherein at least one filter canister is in fluidcommunication with each input feed line and each output feed line.
 13. Ahigh pressure fluid filter as claimed in claim 12, wherein each filtercanister of said plurality of filter canisters comprises a perforatedbase pipe and a media screen.
 14. A high pressure fluid filter asclaimed in claim 12, further comprising a by-pass line in fluidcommunication with said inlet and said outlet.
 15. A high pressure fluidfilter as claimed in claim 12, wherein said plurality of filtercanisters comprise a plurality of media screens, wherein at least two ofsaid plurality of media screens are different.
 16. A process forfiltering fluid for circulation in a well, the process comprising:standing the fluid a first time, whereby debris in the fluid is allowedto settle; filtering under a first pressure the fluid of said standingthe fluid a first time, whereby further debris is removed from thefluid; standing the fluid a second time, whereby further debris isallowed to settle; and filtering the fluid of said standing the fluid asecond time under second pressure, wherein the second pressure is higherthan the first pressure, whereby further debris is removed from thefluid.
 17. A process as claimed in claim 16, wherein said standing thefluid a first time comprises pumping the fluid into a first settlingtank and allowing the fluid to stand substantially still for a period oftime, wherein said standing the fluid a second time comprises pumpingthe fluid into a second settling tank and allowing the fluid to standsubstantially still for a period of time.
 18. A process as claimed inclaim 16, wherein said filtering under a first pressure the fluid ofsaid standing the fluid a first time comprises pumping the fluid througha filter.
 19. A process as claimed in claim 16, wherein said filteringunder a second pressure the fluid of said standing the fluid a secondtime comprises pumping the fluid through a filter.
 20. A process asclaimed in claim 16, further comprising regenerating a filter used insaid filtering under a second pressure.